The purpose of this research is to quantitatively define the role of physical and mechanical parameters in the microcirculation that influence and determine whole organ regulatory and exchange functions. Three basic studies are planned. (1) A comparative study of transcapillary fluid exchange in single capillaries of red versus white skeletal muscle will be conducted. Differences in capillary hydraulic conductivity, transcapillary pressures and surface area will be measured using direct microcirculatory techniques. The purpose of this study is to completely characterize the role of each individual physical determinant of capillary fluid exchange at the microvascular level in terms of functional differences in red versus white skeletal muscle. (2) A second study will be performed to measure the mechanical properties of single, isolated arterioles, in vitro. This work will address two specific questions which directly pertain to the role of physical factors in microvascular function: (a.) Why are there regional differences in vessel reactivity along the length of the vascular tree? (b.) To that extent do the mechanical states of arterioles determine the nature and magnitude of these longitudinal response gradients? (3) The third set of experiments will involve direct measurements of pressure, flows and diameters in the microvasculature of rat intestinal muscle and mucosal villi during venous pressure elevation. The experiments are designed to address the question : Is capillary pressure regulated? The experiments are also designed to specifically test the validity of assumptions and previous interpretations of whole organ experiments which are the foundation for the current concept of myogenic autoregulation in the intestine. The studies will provide detailed information about microvascular function that is necessary to critically test, interpret and evaluate the validity of current concepts about whole organ vascular functions that are not yet clearly understood.